The present invention relates to lubricant compositions and fuel-lubricant mixtures useful in direct fuel injected, crankcase-scavenged two-stroke engines. The use of spark ignited two-stroke internal combustion engines rapidly increased during the last three decades of the twentieth century. This growth, however, has now tapered off, mainly due to the high emissions levels present in these conventional two-stroke engines. The current global population of conventional two-stroke engines is very large, numbering in the tens of millions. Because these two-stroke engines are cost-effective and have high power to weight ratios, they are very commonly used in lawn and garden equipment, portable contractor equipment, such as pumps and electrical generators, low-cost transportation vehicles, such as mopeds, as well as commercial and recreational vehicles including motorcycles, outboard engines, snowmobiles, personal watercraft vehicles and the like.
The increasing need to reduce emissions in these two-stroke engines, especially the larger engines in outboards, snowmobiles, personal watercraft and the like, has led to the incorporation of direct fuel injected technology, where the standard carbureted or electronic fuel injected two-stroke engines are replaced with the new two-stroke direct fuel injection (DFI) engine designs (1998 SAE paper SP-1327). These new DFI engines reduce emissions by directly injecting the fuel into the combustion chamber after the exhaust port is closed, thus eliminating the “short-circuiting” of the fuel/oil mixture, which can be as high as 30%. Since in most two-stroke engines the amount of oil supplied for lubrication is proportional to the fuel consumed, a 30% reduction in fuel consumption may result in a similar reduction of oil for lubrication. This increase in fuel efficiency and the commensurate reduction in fuel flow results in higher operating temperatures, since the cooling effect of excess fuel and air flow is eliminated.
The net result of applying DFI technology to two-stroke engines is that fuel efficiency is improved and emissions are reduced, while maintaining the power density and performance of a conventional two-cycle engine. The challenge is to provide lubricants which will give adequate engine protection at higher operating temperatures and lower oiling rates than the conventional two-stroke engines. Commercial experience over the past few years has demonstrated that many conventional two-stroke oils do not provide adequate performance and protection for this new generation of DFI engines. This lack of oil performance has lead to problems such as scuffing, ring sticking, carbon buildup, power loss, increased emissions, and in some cases, catastrophic engine failure. Carbon build up in the ring grooves is especially prevalent in these engines and causes “ring jacking,” which leads to scuffing and subsequent engine seizure. Ring groove fill ratings, therefore, are an essential measure of differentiating oil performance.
The original equipment manufacturers (OEMs) have addressed some of these problems with engineering solutions, which include special coatings and metallurgy for rings and cylinder walls, redesigned flow and distribution patterns for the fuel, enhanced cooling systems, as well as special injectors for the fuel or fuel/air mixture. These engineering enhancements have reduced but not eliminated the need for special two-stroke lubricants. The design of these special lubricants and their ability to address these problems is further restricted by the fact that these lubricants must burn cleanly. Traditional solutions, such as using higher viscosity lubricants to eliminate scuffing and seizure, may create other problems, such as increased carbon and varnish deposition. Increased deposits lead to problems including spark plug fouling, pre-ignition and ring sticking, which can cause compression loss and subsequent engine failure.
Another traditional solution would be the use of high dosages of nitrogen containing compounds or normal dosages of high-nitrogen compounds. Examples of these solutions would be two widely used OEM oils, designated herein as OEM Oils A and B, which have nitrogen content greater than 0.8% and 1.0% respectively. Although these high-nitrogen packages provide some benefit in the engine, that benefit is mitigated by contributing to heavy carbon deposition in the ring groove area, which then causes ring jacking and subsequent engine seizure.
Some of the problems associated with DFI two-stroke engines have been addressed by including additives commonly associated with four-stroke gasoline and diesel engines; i.e., using zinc dialkyl dithiophosphate (ZDDP) to prevent scuffing and scoring. These traditional additives contain high levels of ash and sulfur/phosphorus chemistry, which are known to cause problems with pre-ignition and deposits. Experience with these problems and some of the attempted solutions support the belief that DFI two-stroke engines benefit from the use of specially formulated lubricants.
The present invention, therefore, describes lubricant formulations that address the special needs of DFI two-stroke engines by maintaining a high level of engine cleanliness and durability at low levels of ash and nitrogen. The low-ash, low-nitrogen formulations described below significantly reduce ring groove fill and heavy carbon deposition, thus eliminating ring jacking and subsequent engine seizure. As a result, these new formulations provide improved engine durability.